1. Field of the Invention
The present invention relates generally to a hydraulically operated braking device for a motor vehicle, and more particularly to such a hydraulic braking device which has a function of automatically regulating the brake fluid pressure in an anti-lock or traction-control manner, or in a manner so as to obtain a desired braking effect.
2. Discussion of the Related Art
In a braking device for an automotive vehicle, the brake fluid pressure or braking pressure is automatically regulated under the control of an electric controller. For example, such an automatic pressure regulating operation is effected during brake application to a wheel in an anti-lock or anti-skid manner so as to avoid an excessive amount or ratio of slip of the wheel, or upon starting or acceleration of the vehicle in a traction-control manner so as to prevent an excessive slip ratio of a drive wheel. The automatic pressure regulation during brake application may also be performed in a manner so as to achieve a desired braking effect exactly corresponding to an operating force or stroke or other parameter indicative of the operating amount or stroke of a brake pedal or other brake operating member, irrespective of a variation in the friction coefficient of a friction member of a brake, a variation in the load of the vehicle, and a variation in the gradient of the road surface on which the vehicle is running.
A braking device having such an automatic pressure regulating function has a high-pressure hydraulic system, a wheel-cylinder hydraulic system, a low-pressure hydraulic system, and a solenoid-operated directional control valve device connected to these hydraulic systems in order to automatically regulate a brake fluid pressure in a wheel cylinder of a brake for a vehicle wheel. The high-pressure hydraulic system includes a hydraulic pressure source such as a master cylinder and a hydraulic pump, and a fluid passage connected to the hydraulic pressure source. The wheel-cylinder hydraulic system includes the wheel cylinder for applying brake to the wheel, and a fluid passage connected to the wheel cylinder. The fluid pressure in the wheel cylinder is controlled directly by the solenoid-operated directional control valve device, or indirectly by the directional control valve device which serves to regulate the pressure of a working fluid of a pressure regulating valve disposed between the wheel cylinder and the directional control valve. The low-pressure hydraulic system usually includes a reservoir for storing under the atmospheric pressure (or a pressure near the atmosphere pressure) the brake fluid, or the working fluid for indirectly regulating the brake fluid pressure, and a fluid passage connected to the reservoir. The solenoid-operated directional control valve device may be a three-position valve which has a pressure-increase position for fluid communication of the wheel-cylinder hydraulic system with the high-pressure hydraulic system, a pressure-decrease position for fluid communication of the wheel-cylinder hydraulic system with the low-pressure hydraulic system, and a pressure-hold position in which the wheel-cylinder hydraulic system is disconnected from both of the high-pressure and low-pressure hydraulic systems. The directional control valve device may be a two-position valve which has the pressure-increase and pressure-decrease positions but does not have a pressure-hold position. Further, the solenoid-operated directional control valve device may be a combination of a plurality of valves, for example, a combination of two solenoid-operated shut-off valves respectively disposed in the high-pressure and low-pressure hydraulic systems, or a combination of a solenoid-operated directional control valve and a solenoid-operated shut-off valve.
In any case, the solenoid-operated directional control valve device is controlled by an electric control device which incorporates a computer. The control device should be adapted to control at least one of an increase rate and decrease rate in the wheel cylinder pressure, so that the wheel cylinder pressure may be optimally regulated. To this end, the solenoid-operated directional control valve device is operated in a duty-cycle control manner wherein the duty cycle of the valve device, e.g., the ratio of the pressure-increase time or pressure-decrease time to the pressure-hold time is properly controlled. Alternatively, a parallel circuit of a fixed flow restrictor and a solenoid operated shut-off valve is provided, so that the rate of flow of the fluid into and from the wheel cylinder is changed in two steps, so that the wheel cylinder pressure may increase and decrease, either slowly or rapidly.
However, such pressure regulating arrangements as described above are not sufficiently satisfactory to accurately and optimally control the wheel cylinder pressure. This is due to the fact that the rate of flow of the fluid through the directional control valve device fluctuates to a considerable extent, with a pressure difference between the upstream and downstream sides of the valve device, upon switching of the valve device to the pressure-increase or pressure-decrease position. In other words, the amount of increase or decrease in the wheel cylinder pressure considerably varies even if the solenoid-operated directional control valve is held in its pressure-increase or pressure-decrease position for a given constant period of time.
In the light of the above, the present applicants developed a braking device as disclosed in Japanese Patent Application No. 62-225860, in which a pilot-operated variable flow restrictor device is provided in one of the high-pressure and wheel-cylinder hydraulic systems. This flow restrictor device is adapted to receive as pilot pressures the pressures in the high-pressure and wheel-cylinder hydraulic systems, so that the rate of fluid flow through the flow restrictor device decreases with a pressure difference between the above two hydraulic systems, in order to adequately regulate the rate of fluid flow from the high-pressure hydraulic system into the wheel cylinder, for controlling the pressure increase in the wheel cylinder.
The applicants recognized a need for improvement of their developed braking device. Described more specifically, the applicants recognized that the amount of pressure increase in the wheel cylinder for a given period of time with the directional control valve placed in its pressure-increase position varies depending not only upon the pressure difference between the high-pressure and wheel-cylinder hydraulic systems, but also upon the pressure in the wheel-cylinder hydraulic system. Thus, the applicants discovered a need of controlling the rate of fluid flow through the flow restrictor valve device, also depending upon the variation in the current pressure in the wheel-cylinder hydraulic system, for accurately regulating the wheel cylinder pressure. This need arises from the fact that the rate of flow of the fluid into the wheel cylinder to obtain a given amount of increase in the wheel cylinder pressure decreases with an increase in the current wheel cylinder pressure, as indicated in the graph of FIG. 7, since the hydraulic circuit through which the fluid is circulated consists of a large number of components, which are made of different materials such as rubber and metals.